The present invention relates to apparatus for elevating personnel and/or loads, utilizing scissors-type linkage.
Personnel elevating apparatus utilizing scissors linkage has long been known. Such apparatus was early developed for use as fire escapes, examples being Von Ehren U.S. Pat. No. 136,883 and Pfautz U.S. Pat. No. 226,101. Currently, and for a number of years, lift apparatus utilizing scissors linkage has found great utility in connection with the elevation of workmen to a position so that they could carry out some desired task.
The workman's platform apparatus which have heretofore been produced have utilized laterally off-set, side-by-side scissor linkages, each scissor linkage having one or more sets of lever arms. The lower ends of the lower lever arms were connected to a base, and the upper ends of the upper lever arms were connected to the elevatable deck; at least one of these arms was pivotally connected to the base, or deck, as the case may be, and usually one arm was pivotally connected to the deck and one arm pivotally connected to the base. The other arm ends were confined to move in a rectilinear manner, that is, with straight line motion, relative to the deck or base with which it was associated, or on which it was mounted. The scissors linkage was actuated by an electric motor or hydraulic cylinder connected to the base and to the linearly movable lever arm end. In some embodiments, in the fully retracted condition, the lever arms were inclined, thereby limiting the minimum height of the workman's platform apparatus. In other embodiments, the lever arms assumed a horizontal position, when the scissor linkage was fully retracted, but such apparatus required not only the noted actuating motor, but one or more additional motors which were utilized to initiate movement of the scissor linkage so that the lever arms were rotated from the horizontal position to an inclined position, following which the main actuating motor was able to move the end of the lever arm to which it was attached, and thereby elevated the deck.
These scissor linkage workman's platform have been found to be objectionable from the point of view of safety, because upon actuation of the scissor linkage, the fingers or limbs of personnel could be caught between the lever arms, as they closed together. This problem has been overcome in the prior art by the provision of lever arms having ends which are off-set, thereby providing a space between serially connected lever arms, to avoid injury to personnel. This is known as the avoidance of "pinch points".
Mankey U.S. Pat. No. 3,350,065 illustrates scissor linkage apparatus which, in the retracted position, has the lever arms substantially parallel and horizontal. A linear actuator extends between pivot pins connecting the upper ends of lower lever arms and the lower ends of upper lever arms, in a horizontal plane. This apparatus has a very low collapsed or retracted height, but in order to extend or elevate the scissor linkage, there are provided cam constructions on the ends of the serially connected lever arms, thereby adding considerably to the expense and complexity of the apparatus, in addition to requiring the elevation of the entire linear actuator. This patent also shows a construction in which the linear actuator is connected to the lower ends of the single set of lever arms, but in this case, the collapsed or retracted height is substantially higher, and the lever arms never are positioned horizontally, but are always inclined.
The above described, currently used scissors linkage workman's platform apparatus necessarily has a non-uniform load applied to the linkage when the deck is elevated, because the deck becomes, in part, cantilevered relative to the supporting linkage. This construction has been utilized, despite disclosures such as found in Boulsover U.S. Pat. No. 3,112,676 and Koehler U.S. Pat. No. 3,823,915; in both of these disclosures, the ends of all of the lever arms which are associated with or connected to the base and the deck are linearly movable, and in addition, there are provided in these disclosures centering links or stays which are pivotally connected to either or both of the base and deck, on the one hand, and to an adjacent lever arm, on the other hand. Thus, these patents disclose constructions which provide uniformity of load distribution from the deck into the linkage, and also enable the deck to be maintained centered relative to the linkage and the linkage maintained centered relative to the base, during extension and retraction of the scissors linkage. However, in Boulsover, the collapsed height of the apparatus is higher than desirable, the arms being at all times inclined, and in Koehler the arms also are inclined in all positions of the apparatus.